hi, I want to sell my iphone, how can I delete all contents? many thanks

settings - general - reset - erase all contents and settings.

Make sure you log off imessage and facetime.

Settings > Messages > iMessage > Off

Settings > FaceTime > Off

Settings > iCloud > Delete Account

Settings > General > Reset > Erase All Content and Settings

Do a factory restore. It will put the state similar as if out of the box. However this will install the latest version of iOS (currently at 6.0.1).